Base Station and Wireless Communication System

ABSTRACT

It is provided a wireless communication system comprising base stations that transmit data to a terminal with cooperation by the base stations. The terminal communicates with the base stations. The terminal periodically transmits, to one of the base stations, information necessary for data transmission from a single base station out of the base stations. Each of the base stations determines whether the terminal needs data transmission through a cooperation among the base stations, and transmits a cooperation information transmission instruction to the terminal, which includes information necessary to execute the data transmission in order to cooperate among the base stations in the case where it is determined that the terminal needs the data transmission through the cooperation among the base stations. The terminal transmits the cooperation information to the base stations in a case of receiving the cooperation information transmission instruction.

BACKGROUND OF THE INVENTION

This invention relates to a wireless communication system which enablesa plurality of base stations to cooperate with one another intransmitting and receiving data to and from at least one wirelesscommunication terminal.

In wireless communication, a wireless communication terminal at the celledge cannot obtain a sufficient user rate because thesignal-to-interference-and-noise ratio (SINR) is deteriorated by theattenuation with distance of a desired wave power from a base station towhich the wireless communication terminal belongs and by the influenceof interference waves from adjacent base stations.

As a technology that solves this problem and improves the user rate of awireless communication terminal at the cell edge, base stationcooperation technology is known in which base stations cooperate withone another in transmitting and receiving data to and from a wirelesscommunication terminal.

The base station cooperation technology continues to be considered forLong Term Evolution (LTE) of 3rd Generation Partnership Project (3GPP)(see, for example, 3GPP TS 36.201 v8.1.0 (2007-11), and 3GPP TS 36.211,TS 36.212, TS 36.213 v8.4.0 (2008-9)), which has been decided all overthe world to be employed as 3.9-generation wireless communicationsystems, and is expected to be incorporated in the standards of LongTerm Evolution-Advanced (LTE-A) (see, for example, 3GPP TR36.814 V0.0.1(2008-9)), which is the successor of LTE and one of the candidates forthe fourth-generation wireless communication systems.

Known concrete data transmission methods in which base stationscooperate with one another include interference avoidance and networkMultiple Input Multiple Output (MIMO) (see, for example, LaurenceMailaender, “Indoor Network MIMO Performance with RegularizedZero-Forcing Transmission,” IEEE ISSSTA 2008, pp. 129-132, August 2008).

Interference avoidance is a technology in which base stations each useBeam Forming (BF) to give high directivity to transmission signals in amanner that prevents signals of adjacent base stations from overlappingone another, thereby avoiding interference and improving SINR.

Network MIMO is an expansion of conventional MIMO transmission, whichuses a plurality of antennas provided in one base station. Network MIMOuses a plurality of antennas provided in a plurality of base stations toperform MIMO transmission.

The following description of this invention focuses on network MIMO. Itshould be noted, however, that this invention is not limited to networkMIMO and is applicable to other methods.

Network MIMO operation described here is classified into single-usertransmission (SU transmission) and multi-user MIMO transmission (MU-MIMOtransmission).

The premise of this invention is that wireless resources are multiplexedby Orthogonal Frequency Division Multiple Access (OFDMA), which isemployed for downlink (downstream) in LTE. However, this invention isnot limited to OFDMA and is applicable to other multiplexing methodssuch as Time Division Multiple Access (TDMA) and Code Division MultipleAccess (CDMA).

In SU transmission, a base station selects one wireless communicationterminal and transmits data to the selected wireless communicationterminal.

A wireless communication terminal in SU transmission receives pilotsignals from a base station to which the wireless communication terminalbelongs and from adjacent base stations, and estimates the channel.

Based on the result of estimating the channel, the wirelesscommunication terminal calculates the quality of the channel to beobtained in the case where network MIMO transmission is used, the numberof MIMO ranks, and a desired precoding matrix.

The wireless communication terminal transmits at least one of theabove-mentioned calculated items and a list of base stations thatparticipate in cooperative transmission to the base station to which thewireless communication terminal belongs, with the use of an uplink(upstream) control signal.

The base station that receives the control signal notifies informationcontained in the received control signal to a cooperation scheduler,which executes wireless resource allocation in base station cooperation.

The cooperation scheduler selects an optimum wireless communicationterminal, a data transmission method, a subcarrier to be used, and thelike based on the notified information, and notifies the selectionresults to the base stations that participate in cooperativetransmission. An optimum wireless communication terminal can be selectedfor each subcarrier of OFDMA.

For example, network MIMO transmission between base stations 1 and 2 andwireless communication terminals 1 and 2 may be carried out such thatthe base stations 1 and 2 transmit by network MIMO transmission to thewireless communication terminal 1 on subcarriers 1 to 12, and to thewireless communication terminal 2 on subcarriers 13 to 24.

Network MIMO transmission methods that can be employed in SUtransmission may include a method that uses Open-Loop MIMO transmissionwhere a wireless communication terminal does not need to specify aprecoding matrix, and the wireless communication terminal uses MinimumMean Square Error (MMSE) or Maximum Likelihood Detection (MLD) as innormal MIMO, a method that uses Closed-Loop MIMO transmission such asEigen Space Division Multiplexing (E-SDM), and a method that is used in,for example, transmit diversity such as Space Time Transmit Diversity(STTD).

In any of the methods given above, base stations participating incooperative transmission exchange data necessary for cooperativetransmission with one another before transmitting to a wirelesscommunication terminal, generate signals in accordance with the employedmethod, and transmit the generated signals to the target wirelesscommunication terminal. The wireless communication terminal decodes thesignals in accordance with the method selected by the base stations, andobtains the data.

The wireless communication terminal that is the target of network MIMOtransmission receives a desired signal from the base stationsparticipating in cooperation, and the channel capacity is thereforemarkedly improved in any of the methods given above.

MU-MIMO transmission is an application of MIMO in which data istransmitted to a plurality of wireless communication terminals.

In MU-MIMO transmission, as in transmission to a single wirelesscommunication terminal, a wireless communication terminal receives pilotsignals from a base station to which the wireless communication terminalbelongs and from adjacent base stations, and estimates the channel.

Based on the result of estimating the channel, the wirelesscommunication terminal calculates the quality of the channel to beobtained in the case where network MIMO transmission is used, the numberof MIMO ranks, a desired precoding matrix, and channel matrices of aplurality of base stations.

The wireless communication terminal transmits at least one of theabove-mentioned calculated items and a list of base stations thatparticipate in cooperative transmission to the base station to which thewireless communication terminal belongs, with the use of an uplinkcontrol signal.

The base station that receives the control signal notifies informationcontained in the received control signal to a cooperation scheduler,which executes wireless resource allocation in base station cooperation.

The cooperation scheduler selects an optimum combination of wirelesscommunication terminals, a data transmission method, a subcarrier to beused, and the like based on the notified information, and notifies theselection results to the base stations that participate in cooperativetransmission. An optimum combination of wireless communication terminalscan be selected for each subcarrier of OFDMA.

For example, network MIMO transmission between base stations 1 and 2 andwireless communication terminals 1, 2 and 3 may be carried out such thatthe base stations 1 and 2 transmit by network MIMO transmission to thewireless communication terminals 1 and 2 on subcarriers 1 to 12, and tothe wireless communication terminals 2 and 3 on subcarriers 13 to 24.

Network MIMO transmission methods that can be employed in MU-MIMOtransmission may include a method that uses Zero Forcing (ZF) where thetransmission side performs precoding with the use of an inverse matrixof a channel matrix, and a method that uses Dirty Paper Coding (DPC)where the channel capacity is improved by utilizing information about aninterference signal.

ZF can be implemented based on a simple principle, but has a problem inthat amplification exceeding the upper limit of transmission power isnecessary depending on the inverse matrix of a channel matrix, whichdegrades the channel capacity.

DPC, on the other hand, is superior to ZF in terms of channel capacitybut has a problem in that the amount of calculation is large. One ofknown DPC implementation methods that alleviate the problem is a methodthat uses LQ decomposition.

In the method that uses LQ decomposition, a channel matrix is decomposedinto a lower triangular matrix and a product of unitary matrices. Thetransmission side executes advance equalization processing based on thelower triangular matrix, and executes precoding through Hermitiantransposition of the unitary matrices. This procedure requires an amountof calculation that can be implemented in practice and, because unitarymatrices are used in precoding, does not cause the extreme amplificationof signal amplitude as in ZF. As a result, interference from adjacentcells is cancelled in a wireless communication terminal and the channelcapacity is therefore improved.

SUMMARY OF THE INVENTION

In the case of MIMO transmission in a wireless communication systemwhere base stations cooperate with one another to transmit data, awireless communication terminal needs to periodically transmit thequality of a channel between the wireless communication terminal and abase station to which the wireless communication terminal belongs, thequality of the channel in cooperative transmission, the number of MIMOranks, and an index of a desired precoding matrix to the base stationwith the use of uplink wireless resources.

When the MIMO transmission is transmission to a plurality of wirelesscommunication terminals, the wireless communication terminal furtherneeds to transmit a matrix of channels between the wirelesscommunication terminal and all base stations participating incooperative transmission to the base station with the use of uplinkwireless resources.

In the case of multicarrier transmission such as OFDMA, considering theinfluence of frequency-selective fading, information necessary for basestation cooperation is desirably transmitted to base stations on asubband-basis as described above.

A subband here means a band formed by bundling a plurality ofconsecutive subcarriers together, and the entire band of the system canbe divided into a plurality of subbands. At least one resource block ispresent in a single subband.

Each wireless communication terminal spends uplink wireless resources totransmit information necessary for base station cooperation to the basestation as described above. Accordingly, an increase in the number ofwireless communication terminals belonging to the base station means alarger consumption of uplink wireless resources for the transmission ofinformation necessary for base station cooperation, and cuts into uplinkwireless resources that are used for user data transmission. This is aserious problem when the increasing popularity of such applications asIP phone and video uploading is taken into account.

Another problem of data transmission in which base stations cooperatewith one another is that the amount of calculation for allocatingwireless resources, namely, the processing amount of the cooperationscheduler, increases.

Take as an example the allocation of wireless resources to one resourceblock. The term resource block here means the unit by which wirelessresources are allocated, and each resource block is an aggregation ofconsecutive subcarriers.

In non-cooperative data transmission which involves a single basestation, the cooperation scheduler selects for SU transmission anoptimum wireless communication terminal from among wirelesscommunication terminals that belong to the base station, and selects forMU-MIMO transmission an optimum combination of wireless communicationterminals from among the wireless communication terminals that belong tothe base station.

In data transmission where base stations cooperate with one another, thecooperation scheduler needs to select an optimum wireless communicationterminal, or an optimum combination of wireless communication terminals,from among all wireless communication terminals that belong to therespective base stations participating in the cooperation.

Further, there is a plurality of possible combinations concerning whichbase stations are to participate in the cooperation, which of thecooperation methods described above is to be employed, and the like. Theamount of calculation required for wireless resource allocation istherefore larger in cooperative transmission than in transmission from asingle base station.

The increase in the amount of calculation for wireless resourceallocation is a huge problem in putting cooperative transmission intopractice because the actual number of resource blocks is large (forexample, a number of the maximum resource block in LTE is 110).

To summarize, data transmission in which base stations cooperate withone another has the following two problems. Firstly, the transmission ofinformation necessary for base station cooperation from a wirelesscommunication terminal to a base station cuts into other uses of uplinkwireless resources. Secondly, the cooperation scheduler needs to handlea larger amount of calculation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sequence diagram illustrating processing that is executedafter the sequence of FIG. 13, form a reception of user data destined tothe wireless communication terminal by the base station from the gatewaydevice, to a cooperative transmission through MU-MIMO transmissionaccording to the first embodiment of this invention.

FIG. 2 is a diagram illustrating the configuration of a networkaccording to the first embodiment of this invention.

FIG. 3 is a block diagram illustrating the configuration of a basestation according to the first embodiment of this invention.

FIG. 4 is a block diagram illustrating the configuration of a wirelesscommunication terminal according to the first embodiment of thisinvention.

FIG. 5A is a block diagram illustrating the configuration of thecooperation scheduler according to the first embodiment of thisinvention.

FIG. 5B is a diagram illustrating a database included in the cooperationscheduler according to the first embodiment of this invention.

FIG. 6 is a flow chart illustrating processing that is executed when thein-station scheduler receives a scheduling request from the data signalprocessing module or the control signal processing module according tothe first embodiment of this invention.

FIG. 7A is a flow chart illustrating processing that is executed whenthe control signal processing module receives a request to transmit acooperation information request signal from the in-station scheduleraccording to the first embodiment of this invention.

FIG. 7B is a flow chart illustrating processing that is executed whenthe control signal processing module receives a resource allocationresult from the in-station scheduler according to the first embodimentof this invention.

FIG. 7C is a flow chart illustrating processing that is executed whenthe control signal processing module receives a resource allocationrequest signal from the wireless communication terminal according to thefirst embodiment of this invention.

FIG. 8A is a flow chart illustrating processing that is executed whenthe data signal processing module receives a resource allocation resultfrom the in-station scheduler according to the first embodiment of thisinvention.

FIG. 8B is a flow chart illustrating processing that is executed whenthe data signal processing module receives user data that is destined toone of the wireless communication terminals from the gateway deviceaccording to the first embodiment of this invention.

FIG. 9A is a flow chart illustrating processing that is executed whenthe control signal processing module receives a resource allocationsignal from the base station according to the first embodiment of thisinvention.

FIG. 9B is a flow chart illustrating processing that is executed whenthe control signal processing module receives a cooperation informationrequest signal from the base stations according to the first embodimentof this invention.

FIG. 10 is a flow chart illustrating processing that is executed whenthe cooperation scheduler receives a cooperation scheduling request fromthe in-station scheduler according to the first embodiment of thisinvention.

FIG. 11A is a diagram illustrating the packet format of the cooperationinformation request signal according to the first embodiment of thisinvention.

FIG. 11B is a diagram illustrating the packet format of the resourceallocation signal according to the first embodiment of this invention.

FIG. 12A is a diagram illustrating the packet format of a cooperationinformation notification signal for Open-Loop MIMO according to thefirst embodiment of this invention.

FIG. 12B is a diagram illustrating the packet format of a cooperationinformation notification signal for MU-MIMO according to the firstembodiment of this invention.

FIG. 13 is a sequence diagram illustrating processing form a receptionof pilot signals by the wireless communication terminal from the basestation, to a transmission of information necessary for single-basestation transmission by the wireless communication terminal to the basestation according to the first embodiment of this invention.

FIG. 14 is a sequence diagram illustrating processing form the receptionof user data destined to the wireless communication terminal by the basestation from the gateway device, to cooperative transmission through SUtransmission according to the second embodiment of this invention.

FIG. 15 is a sequence diagram illustrating processing that is executedin the case where the base station request information necessary forcooperative transmission before user data arrives at the wirelesscommunication terminal according to the third embodiment of thisinvention.

FIG. 16 is a sequence diagram illustrating processing that is executedwhen the wireless communication terminal that has been at the cellcenter at first and has used single-base station transmission moves tothe cell edge and comes to need cooperative transmission according tothe fourth embodiment of this invention.

FIG. 17 is a sequence diagram illustrating processing according to thefifth embodiment of this invention that is executed in the case wherethe wireless communication terminal that has at the cell edge at firstand has used data transmission by cooperating transmission moves to thecell center and switches to single-base station transmission.

FIG. 18 is a diagram for explaining a resource grid upon cooperativetransmission according to the third embodiment of this invention.

FIG. 19A is a diagram illustrating subbands according to the sixthembodiment of this invention.

FIG. 19B is a diagram illustrating an association relation between anumber of the resource block and a number of the subband according tothe sixth embodiment of this invention.

FIG. 20 is a diagram illustrating effects according to the firstembodiment or the second embodiment of this invention.

FIG. 21A is a graph illustrating changes in the number of bits in anuplink wireless resource band that is used by one wireless communicationterminal for one feedback transmission session in the case where thecooperating wireless communication terminal ratio is changed based onthe packet format of the cooperation information notification signal ofFIG. 12A.

FIG. 21B is a graph illustrating changes in the number of bits in anuplink wireless resource band that is used by one wireless communicationterminal for one feedback transmission session in the case where thecooperating wireless communication terminal ratio is changed based onthe packet format of the cooperation information notification signal ofFIG. 12B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The premise of description given below on embodiments of this inventionis that the choice of multiplexing method follows the example of 3GPPLTE, specifically, OFDMA for downlink data transmission and SingleCarrier-Frequency Division Multiple Access (SC-FDMA) for uplink datatransmission. However, this invention is not limited to a wirelesscommunication system that uses these methods, and is applicable to othermultiplexing methods including CDMA and TDMA.

First Embodiment

In a first embodiment of this invention, a base station requestsinformation necessary for cooperative transmission (hereinafter,referred to as cooperation information) from a wireless communicationterminal that has failed to be allocated a wireless resource throughsingle-base station transmission, a cooperation scheduler refers to thecollected cooperation information and decides on MU-MIMO transmission,and the base station executes MU-MIMO transmission following thedecision. This series of processing steps is described below.

Described first are the overall network configuration and theconfigurations of a base station and a wireless communication terminal.

FIG. 2 is a diagram illustrating the configuration of a networkaccording to the first embodiment of this invention.

The network includes a core network 1, a gateway device 2, base stations100, and a cooperation scheduler 190.

The base stations 100 each constitute a cell. Wireless communicationterminals 200 are scattered within each cell constituted of one basestation 100, and each wireless communication terminal 200 belongs to oneof the base stations 100.

The base stations 100 are connected to the core network 1 via thegateway device 2. In the first embodiment of this invention, the basestations 100 and the gateway device 2 are connected by cables with theuse of optical fibers or the like. It should be noted that theconnection between the base station 100 and the gateway device 2 may bewireless.

The cooperation scheduler 190 executes wireless resource allocation indata transmission where the base stations 100 cooperate with one another(hereinafter, referred to as cooperative transmission).

In the first embodiment of this invention, each base station 100 has acommunication interface (IF) for communicating with the cooperationscheduler 190.

This invention does not depend on where the cooperation scheduler 190 isset up. For instance, the cooperation scheduler 190 may be set up as anindependent device or may be contained in each base station 100 or inthe gateway device 2. In the following description of the firstembodiment of this invention, where the cooperation scheduler 190 is setup is not specified.

FIG. 3 is a block diagram illustrating the configuration of each basestation 100 according to the first embodiment of this invention.

The base station 100 includes a data signal processing module 101, acontrol signal processing module 102, an RF processing module 103, anantenna 104, an in-station scheduler 105, a cooperation schedulerinterface (IF) 106, a cooperating base station interface (IF) 107, asignal transmitter 110, a signal receiver 120, a channel estimationmodule 131, and a CQI/PMI/RI calculating module 132.

The signal transmitter 110 includes encoders 111, modulators 112, alayer mapping/precoding module 113, subcarrier mapping modules 114,pilot inserting modules 115, and OFDM modulators 116.

The signal receiver 120 includes decoders 121, demodulators 122, anlayer demapping module 123, subcarrier demapping modules 124, an MIMOreceiver 125, and SC-FDMA demodulators 126.

Processing of the respective components is described below.

User data destined to one of the wireless communication terminals 200 isreceived from the gateway device 2, accumulated in a buffer of the datasignal processing module 101, allocated a wireless resource, and thensent to the signal transmitter 110.

User data sent from the signal receiver 120 to the data signalprocessing module 101 is transmitted to the core network 1 via thegateway device 2.

The control signal processing module 102 transmits a control signalbetween the base station 100 and the wireless communication signals 200via the signal transmitting module 110, or receives the control signalvia the signal receiver 120, as the need arises.

The in-station scheduler 105 executes wireless resource allocation forsingle-base station transmission, performs scheduling based oninformation received from the data signal processing module 101 and thecontrol signal processing module 102, and notifies the result of thescheduling to the data signal processing module 101 and the controlsignal processing module 102.

The in-station scheduler 105 also communicates with the cooperationscheduler 190 via the cooperation scheduler IF 106 in order to implementcooperative transmission.

The data signal processing module 101 communicates with other basestations 100 participating in cooperative transmission via thecooperating base station IF 107.

When the signal transmitter 110 receives a data signal from the datasignal processing module 101 or a control signal from the control signalprocessing module 102, one of the encoders 111 generate a code word byattaching a cyclic redundancy code (CRC) to the received data signal orcontrol signal and subsequently performing error correcting codingprocessing with the use of a turbo code, a convolutional code, or thelike.

The modulators 112 each execute appropriate modulation to generate amodulation symbol sequence from a generated code word.

The layer mapping/precoding module 113 executes layer mapping processingfor accomplishing antenna diversity and precoding processing forimproving the reception precision of the wireless communicationterminals 200.

The subcarrier mapping modules 114 each allocate each symbol in a symbolseries input from the layer mapping/precoding module 113 to one ofsubcarriers contained in an arbitrary OFDM symbol.

The pilot inserting modules 115 each insert a pilot symbol, which isused by the wireless communication terminals 200 to estimate thedownlink channel, in an appropriate place.

The OFDM modulators 116 each execute inverse discrete Fourier transform(IDFT) processing and cyclic prefix (CP) insertion, and output abaseband OFDM signal.

The output baseband OFDM signal is transmitted to the RF processingmodule 103, which executes digital-analog conversion, upconverting, andamplification processing for each signal separately, and the signalwhich has been subjected to the processing given above is transmittedfrom the antenna 104 to the wireless communication terminals 200.

Meanwhile, signals received by the antenna 104 from the wirelesscommunication terminals 200 are sent to the RF processing module 103,which executes amplification processing, downconverting, andanalog-digital conversion processing for each signal. The signals whichhave been subjected to the processing given above are transmitted to theSC-FDMA demodulators 126.

The SC-FDMA demodulators 126 each execute CP removal, DFT processing,and IDFT processing for SC-FDMA reception for a signal received from theRF processing module 103.

A pilot signal part of the signal which has been subjected to theprocessing given above is transmitted to the channel estimation module131, and the rest of the signal is input to the MIMO receiver 125.

The channel estimation module 131 estimates the uplink channel based onthe received pilot signal, and transmits the estimated channel matrix tothe MIMO receiver 125 and the CQI/PMI/RI calculating module 132.

Based on the received channel matrix, the MIMO receiver 125 executesMIMO reception processing with the use of MMSE and MLD for an input fromone of the SC-FDMA demodulators 126, and transmits layer-based outputsto the subcarrier demapping modules 124.

The subcarrier demapping modules 124 execute processing reverse toprocessing executed by subcarrier mapping modules 214 (illustrated inFIG. 4), which are included in each wireless communication terminal 200.Specifically, the subcarrier demapping modules 124 each generate areception symbol sequence from a subcarrier contained in an arbitraryOFDMA symbol, and output the generated reception symbol sequence to thelayer demapping module 123.

The layer demapping module 123 executes processing reverse to layermapping processing executed by a layer mapping/precoding module 213(illustrated in FIG. 4), which is included in each wirelesscommunication terminal 200.

The demodulators 122 each execute demodulation processing for areception symbol sequence, and outputs a log likelihood ratio sequenceto one of the decoders 121.

The decoders 121 each execute error correcting decoding processing andCRC check processing for a log likelihood ratio sequence that has beeninput, and notify the result of the CRC check to the control signalprocessing module 102.

When the CRC check resulted in a success, a user data signal istransmitted to the data signal processing module 101, and the controlsignal is transmitted to the control signal processing module 102.

The CQI/PMI/RI calculating module 132 calculates Channel QualityIndication (CQI, the channel quality), Precoding Matrix Indicator (PMI,a precoding matrix desired by the wireless communication terminal), andRank Indication (RI, the rank in MIMO transmission) for the uplink,based on a received channel matrix, and notifies calculation results tothe control signal processing module 102.

Details of processing executed by the components of the base station 100are described next.

FIG. 6 is a flow chart illustrating processing that is executed when thein-station scheduler 105 receives a scheduling request from the datasignal processing module 101 or the control signal processing module 102according to the first embodiment of this invention.

The in-station scheduler 105 receives a scheduling request (301) anddetermines whether the received scheduling request is a downlinkscheduling request or an uplink scheduling request (302).

The received scheduling request contains information for identifyingwhether the scheduling request is for uplink or downlink, and thein-station scheduler 105 refers to this information to determine.

When it is determined that the scheduling request is for downlink, thein-station scheduler 105 allocates wireless resources for single-basestation transmission, based on data information to be transmitted and onthe Channel Quality Indication (CQI, the channel quality), PrecodingMatrix Indicator (PMI, a precoding matrix desired by the wirelesscommunication terminal), and Rank Indication (RI, the rank in MIMOtransmission) of a downlink to one of the wireless communicationterminals 200 that is the destination of the data information (303).Data information to be transmitted is, for example, the amount of dataor a request for the quality of service (QoS) such as delay of data.

The in-station scheduler 105 determines whether or not the wirelessresource allocation for single-base station transmission has succeededor not (304).

When the wireless resource allocation for single-base stationtransmission is determined as a success, the in-station scheduler 105notifies the control signal processing module 102 and the data signalprocessing module 101 of the result of the resource allocation (311 and312), and ends the processing (313).

When the wireless resource allocation for single-base stationtransmission is determined as a failure, the in-station scheduler 105decides that cooperative transmission is necessary, and requests thecontrol signal processing module 102 to transmit a signal that requestsinformation necessary for cooperation (hereinafter, referred to ascooperation information request signal) to the wireless communicationterminal 200 that has failed to be allocated a wireless resource throughsingle-base station transmission (306).

The in-station scheduler 105 receives from the control signal processingmodule 102 a request for wireless resource allocation for cooperativetransmission (hereinafter, referred to as cooperation scheduling) (see324 of FIG. 7A) (307).

Receiving the cooperation scheduling request, the in-station scheduler105 requests cooperation scheduling from the cooperation scheduler 190via the cooperation scheduler IF 106.

The in-station scheduler 105 receives a cooperation scheduling result(see 409 of FIG. 10) from the cooperation scheduler 190 (309), notifiesthe received result to the control signal processing module 102 and thedata signal processing module 101 (311 and 312), and ends the processing(313).

When it is determined in Step 302 that the received scheduling requestis for uplink, the in-station scheduler 105 allocates uplink wirelessresources to the relevant wireless communication terminal 200 based ondata information to be received and on the CQI, PMI, and RI of an uplinkfrom the wireless communication terminal 200 (310).

The in-station scheduler 105 subsequently executes the same processingas in the case of a downlink scheduling request, notifies a resourceallocation result to the control signal processing module 102 and thedata signal processing module 101 (311 and 312), and ends the processing(313).

FIG. 7A is a flow chart illustrating processing that is executed whenthe control signal processing module 102 receives a request to transmita cooperation information request signal from the in-station scheduler105 according to the first embodiment of this invention.

The control signal processing module 102 receives from the in-stationscheduler 105 a request to transmit a cooperation information requestsignal (see 306 of FIG. 6) (321), generates a cooperation informationrequest signal, and transmits the generated cooperation informationrequest signal to the relevant wireless communication terminal 200(322). Details of the packet format of the cooperation informationrequest signal are described later with reference to FIG. 11A.

The control signal processing module 102 receives a cooperationinformation notification signal (see 390 of FIG. 9B) from the wirelesscommunication terminal 200 (323). The cooperation informationnotification signal is a signal for notifying to the base station 100information necessary to execute wireless resource allocation forcooperative transmission. The cooperation information notificationsignal contains a cooperative transmission method desired by thewireless communication terminal 200, a list of the base stations 100that participate in the cooperative transmission, and at least one typeof information out of CQI and a channel matrix in cooperativetransmission. Details of the packet format of the cooperationinformation notification signal are described later with reference toFIGS. 12A and 12B.

The control signal processing module 102 issues a request forcooperation scheduling to the in-station scheduler 105 (324), and endsthe processing (325). This request contains the obtained cooperationinformation.

FIG. 7B is a flow chart illustrating processing that is executed whenthe control signal processing module 102 receives a resource allocationresult from the in-station scheduler 105 according to the firstembodiment of this invention.

The control signal processing module 102 receives a resource allocationresult (see 311 of FIG. 6) from the in-station scheduler 105 (331), andrepeatedly executes the following processing steps until every wirelesscommunication terminal 200 that is specified in the received resourceallocation result is processed (332). Specifically, the control signalprocessing module 102 selects one wireless communication terminal 200from among the wireless communication terminals 200 that are specifiedin the received resource allocation result, and executes the followingprocessing steps.

The control signal processing module 102 first determines whether or nota wireless resource has been allocated to the selected wirelesscommunication terminal 200 (333).

When it is determined that the selected wireless communication terminal200 has been allocated a wireless resource, the control signalprocessing module 102 generates a resource allocation signal (334). Theresource allocation signal is a signal for notifying the wirelesscommunication terminal 200 of a wireless resource that has beenallocated. Details of the packet format of the resource allocationsignal are described later with reference to FIG. 11B.

When it is determined that the selected wireless communication terminal200 has not been allocated a wireless resource, the control signalprocessing module 102 holds off data transmission to this wirelesscommunication terminal 200 until the next transmission timing (335). Thenext transmission timing may be, in the case of LTE, for example, thenext time a subframe is transmitted.

The control signal processing module 102 determines whether or not everywireless communication terminal 200 specified in the received resourceallocation result has been processed (336).

When it is determined that not every wireless communication terminal 200specified in the received resource allocation result has been processed,the control signal processing module 102 returns to Step 332 to repeatthe subsequent processing steps.

When it is determined that every wireless communication terminal 200specified in the received resource allocation result has been processed,the control signal processing module 102 transmits generated resourceallocation signals to the wireless communication terminals 200 (337),and ends the processing (338).

FIG. 7C is a flow chart illustrating processing that is executed whenthe control signal processing module 102 receives a resource allocationrequest signal from one of the wireless communication terminals 200according to the first embodiment of this invention.

The control signal processing module 102 receives a resource allocationrequest signal from one of the wireless communication terminals 200(341), requests uplink scheduling from the in-station scheduler 105(342), and ends the processing (343).

FIG. 8A is a flow chart illustrating processing that is executed whenthe data signal processing module 101 receives a resource allocationresult from the in-station scheduler 105 according to the firstembodiment of this invention.

The data signal processing module 101 receives a resource allocationresult from the in-station scheduler 105 (351) and determines whetherthe resource allocation is downlink resource allocation or uplinkresource allocation (352).

When it is determined that the resource allocation is for downlink, thedata signal processing module 101 refers to the resource allocationresult to determine whether or not to perform cooperative transmission(353).

When it is determined that cooperative transmission is not to beperformed, the data signal processing module 101 proceeds to Step 355.

When it is determined that cooperative transmission is to be performed,the data signal processing module 101 exchanges necessary user data withother base stations 100 that participate in the cooperative transmission(354), and proceeds to Step 355.

The data signal processing module 101 next transfers necessary data fromthe buffer in the data signal processing module 101 to the signaltransmitting module 110 (355), and ends the processing (357).

When it is determined in Step 352 that the resource allocation is foruplink, the data signal processing module 101 secures a buffer area inpreparation for data reception (356), and ends the processing (357).

FIG. 8B is a flow chart illustrating processing that is executed whenthe data signal processing module 101 receives user data that isdestined to one of the wireless communication terminals 200 from thegateway device 2 according to the first embodiment of this invention.

The data signal processing module 101 receives from the gateway device 2user data destined to one of the wireless communication terminals 200(361), requests downlink scheduling from the in-station scheduler 105(362), and ends the processing (363).

The configuration of each wireless communication terminal 200 isdescribed next.

FIG. 4 is a block diagram illustrating the configuration of eachwireless communication terminal 200 according to the first embodiment ofthis invention.

The wireless communication terminal 200 includes a data signalprocessing module 201, a control signal processing module 202, an RFprocessing module 203, an antenna 204, a signal transmitter 210, asignal receiver 220, a channel estimation module 231, and a CQI/PMI/RIcalculating module 232.

The signal transmitter 210 includes encoders 211, modulators 212, thelayer mapping/precoding module 213, the subcarrier mapping modules 214,pilot inserting modules 215, and SC-FDMA modulators 216.

The signal receiver 220 includes decoders 221, demodulators 222, anlayer demapping module 223, subcarrier demapping modules 224, an MIMOreceiver 225, and OFDM demodulators 226.

Processing of the respective components is described below.

User data that is generated by an upper layer in the wirelesscommunication terminal 200, such as the Medium Access Control (MAC)layer, is accumulated in a buffer in the data signal processing module201, allocated wireless resources, and then transmitted to the signaltransmitter 210.

User data that is sent from the signal receiver 220 to the data signalprocessing module 201 is handed over to an upper layer.

The control signal processing module 202 transmits a control signalbetween one of the base stations 100 and the wireless communicationterminal 200 via the signal transmitter 210, or receives the controlsignal via the signal receiver 220, as the need arises.

When the signal transmitter 210 receives a data signal from the datasignal processing module 201 or a control signal from the control signalprocessing module 202, one of the encoders 211 generate a code word byattaching the CRC to the received data signal or control signal andsubsequently performing error correcting coding processing with the useof a turbo code, a convolutional code, or the like.

The modulators 212 each execute appropriate modulation to generate amodulation symbol sequence from a generated code word.

The layer mapping/precoding module 213 executes layer mapping processingfor accomplishing antenna diversity and precoding processing forimproving the reception precision of the base station 100.

The subcarrier mapping module s 214 each allocate each symbol in asymbol series input from the layer mapping/precoding module 213 to oneof subcarriers contained in an arbitrary SC-FDMA symbol.

The pilot inserting modules 215 each insert a pilot symbol, which isused by the base station 100 to estimate the uplink channel, in anappropriate place.

The SC-FDMA modulators 216 each execute DFT processing, IDFT processing,and CP insertion for SC-FDMA, and output a baseband SC-FDMA signal.

The output baseband SC-FDMA signal is transmitted to the RF processingmodule 203, which executes digital-analog conversion, upconverting, andamplification processing for each signal separately, and the signalwhich has been subjected to the processing given above is transmittedfrom the antenna 104 to the base station 100.

Meanwhile, signals received by the antenna 204 from the base station 100are sent to the RF processing module 203, which executes amplificationprocessing, downconverting, and analog-digital conversion processing foreach signal. The signals which have been subjected to the processinggiven above are transmitted to the OFDM demodulators 226.

The OFDM demodulators 226 each execute CP removal and DFT processing fora signal received from the RF processing module 203.

A pilot signal part of the signal which has been subjected to theprocessing given above is transmitted to the channel estimation module231, and the rest of the signal is input to the MIMO receiver 225.

The channel estimation module 231 estimates the downlink channel basedon the received pilot signal, and transmits the estimated channel matrixto the MIMO receiver 225 and the CQI/PMI/RI calculating module 232.

Based on the received channel matrix, the MIMO receiver 225 executesMIMO reception processing with the use of MMSE and MLD for an input fromone of the OFDM demodulators 226, and transmits layer-based outputs tothe subcarrier demapping modules 224.

The subcarrier demapping modules 224 execute processing reverse to theprocessing executed by the subcarrier mapping modules 114, which areincluded in each base station 100. Specifically, the subcarrierdemapping modules 224 each generate a reception symbol sequence from asubcarrier contained in an arbitrary SC-FDMA symbol, and output thegenerated reception symbol sequence to the layer demapping module 223.

The layer demapping module 223 executes processing reverse to the layermapping processing executed by the layer mapping/precoding module 113,which is included in each base station 100.

The demodulators 222 each execute demodulation processing for areception symbol sequence, and outputs a log likelihood ratio sequenceto one of the decoders 221.

The decoders 221 each execute error correcting decoding processing andCRC check processing for a log likelihood ratio sequence that has beeninput, and notify the result of the CRC check to the control signalprocessing module 202.

When the CRC check resulted in a success, a user data signal istransmitted to the data signal processing module 201, and the controlsignal is transmitted to the control signal processing module 202.

The CQI/PMI/RI calculating module 232 calculates CQI, PMI, and RI forthe downlink, based on a received channel matrix, and notifiescalculation results to the control signal processing module 202.

Details of processing executed by the components of the wirelesscommunication terminals 200 are described next.

FIG. 9A is a flow chart illustrating processing according to the firstembodiment of this invention that is executed when the control signalprocessing module 202 receives a resource allocation signal from one ofthe base stations 100.

The control signal processing module 202 receives a resource allocationsignal from one of the base stations 100 (371), and determines whetherthe received resource allocation signal is about downlink resourceallocation or uplink resource allocation (372).

When it is determined that the received resource allocation signal isabout downlink resource allocation, the control signal processing module202 notifies the data signal processing module 201 of the need toprepare for data reception (373), and ends the processing (375).

When it is determined that the received resource allocation signal isabout uplink resource allocation, the control signal processing module202 requests the data signal processing module 201 to transmit data(374), and ends the processing (375).

FIG. 9B is a flow chart illustrating processing according to the firstembodiment of this invention that is executed when the control signalprocessing module 202 receives a cooperation information request signalfrom one of the base stations 100.

In this flow chart, cooperation information is generated for everytarget subband to transmit a cooperation information notificationsignal.

The control signal processing module 202 receives a cooperationinformation request signal from one of the base stations 100 (381), andrepeatedly executes the following processing steps until every targetsubband is processed (382). Specifically, the control signal processingmodule 202 selects one subband from among all target subbands, andexecutes the following processing steps.

The control signal processing module 202 determines whether or not touse MU-MIMO transmission as the transmission method to be used incooperative transmission (383).

For example, in the case where the cooperation information requestsignal contains a field that indicates a transmission method to be usedin cooperative transmission, the control signal processing module 202determines whether or not to use MU-MIMO transmission by referring tothis field. Alternatively, whether or not to use MU-MIMO transmissionmay be determined by the wireless communication terminal 200 based on achannel matrix of the downlink and on settings of the wirelesscommunication terminal 200 itself.

When it is determined that MU-MIMO transmission is to be used as thetransmission method of the cooperative transmission, the control signalprocessing module 202 obtains a channel matrix from every base station100 that participates in the cooperative transmission (384).

When it is determined that MU-MIMO transmission is not to be used as thetransmission method of the cooperative transmission, the control signalprocessing module 202 proceeds to Step 385.

The control signal processing module 202 next determines whether or notto use Closed-Loop transmission as the transmission method of thecooperative transmission (385). As in Step 383, whether or not to useClosed-Loop transmission is determined by referring to a field in thecooperation information request signal, or based on a determinationresult of the wireless communication terminal 200.

When it is determined that Closed-Loop transmission is to be used as thetransmission method of the cooperative transmission, the control signalprocessing module 202 obtains CQI, PMI, and RI that are based on theemployed transmission method from the CQI/PMI/RI calculating module 232(386).

When it is determined that Closed-Loop transmission is not to be used asthe transmission method of the cooperative transmission, the controlsignal processing module 202 proceeds to Step 387.

The control signal processing module 202 next determines whether or notto use Open-Loop transmission as the transmission method of thecooperative transmission (387). As in Step 383, whether or not to useOpen-Loop transmission is determined by referring to a field in thecooperation information request signal, or based on a determinationresult of the wireless communication terminal 200.

When it is determined that Open-Loop transmission is to be used as thetransmission method of the cooperative transmission, the control signalprocessing module 202 obtains CQI and RI that are based on the employedtransmission method from the CQI/PMI/RI calculating module 232 (388).

When it is determined that Open-Loop transmission is not to be used asthe transmission method of the cooperative transmission, the controlsignal processing module 202 proceeds to Step 389.

The control signal processing module 202 next determines whether or notevery target subband has been processed (389).

When it is determined that not every target subband has been processed,the control signal processing module 202 returns to Step 382 to repeatthe subsequent processing steps.

When it is determined that every target subband has been processed, thecontrol signal processing module 202 generates a cooperation informationnotification signal based on the obtained cooperation information,transmits the generated cooperation information notification signal tothe base station 100 (390), and ends the processing (391).

The cooperation scheduler 190 is described next.

FIG. 5A is a block diagram illustrating the configuration of thecooperation scheduler 190 according to the first embodiment of thisinvention.

The cooperation scheduler 190 is connected to the cooperation schedulerIF 106 of each base station 100 via one of IFs 191 to communicate withthe in-station scheduler 105. The connection between the IFs 191 and thecooperation scheduler IFs 106 may be cable connection or may bewireless.

FIG. 5B is a diagram illustrating a database included in the cooperationscheduler 190 according to the first embodiment of this invention.

The cooperation scheduler 190 refers to this database, which is denotedby 550, when allocating wireless resources.

The database 550 contains in each entry a terminal 551, a data arrivaltime 552, a data amount 553, and cooperation instantaneous throughput554, and average throughput 555.

The terminal 551 stores an identifier for uniquely identifying eachwireless communication terminal 200. Stored as the data arrival time 552is a time at which user data destined to the wireless communicationterminal 200 that is associated with the terminal 551 arrives at therelevant base station 100.

The data amount 553 includes the data amount of the user data destinedto the wireless communication terminal 200 that is associated with theterminal 551. The cooperation instantaneous throughput 554 includes theinstantaneous throughput in cooperative transmission. The averagethroughput 555 includes the average throughput in the transmission ofuser data destined to the wireless communication terminal 200 that isassociated with the terminal 551.

The cooperation scheduler 190 calculates the amount of wireless resourcenecessary to transmit user data based on the data arrival time 552 andthe data amount 553, and determines the priority of each wirelesscommunication terminal 200 based on the cooperation instantaneousthroughput 554 and the average throughput 555.

The priority may be determined by a method that uses proportionalfairness to select first the wireless communication terminal 200 thathas the largest quotient of the cooperation instantaneous throughput 554divided by the average throughput 555. The database 550 of FIG. 5B is anexample, and the database 550 may have other configurations.

FIG. 10 is a flow chart illustrating processing according to the firstembodiment of this invention that is executed when the cooperationscheduler 190 receives a cooperation scheduling request from thein-station scheduler 105.

The cooperation scheduling 190 receives a cooperation scheduling requestfrom the in-station scheduler 105 (401), and selects at least onewireless communication terminal 200 that has high priority (402). Theselection may be made by, for example, a method that uses proportionalfairness and the database of FIG. 5B.

The cooperation scheduler 190 repeatedly executes the followingprocessing steps until every selected wireless communication terminal200 is processed (400). Specifically, the cooperation scheduling 190selects one wireless communication terminal 200 from among the wirelesscommunication terminals 200 that have been selected in Step 402, andexecutes the following processing steps.

The cooperation scheduling 190 first refers to the database of FIG. 5Bto determine whether or not MU-MIMO transmission is possible (404).

When it is determined that MU-MIMO transmission is possible, thecooperation scheduler 190 allocates wireless resources for MU-MIMOtransmission (407) and proceeds to Step 408.

When it is determined that MU-MIMO transmission is not possible, thecooperation scheduler 190 determines whether or not SU transmission ispossible (405).

When it is determined that SU transmission is possible, the cooperationscheduler 190 allocates wireless resources for SU transmission (406) andproceeds to Step 408.

When it is determined that SU transmission is not possible, thecooperation scheduler 190 proceeds to Step 408.

The cooperation scheduler 190 determines whether or not every wirelesscommunication terminal 200 selected in Step 402 has been processed, orwhether or not every wireless resource that is available for allocationhas been put into use (408).

When it is determined that the criterion given above is not satisfied,the cooperation scheduler 190 returns to Step 402 to repeat thesubsequent processing steps.

When it is determined that the criterion given above is satisfied, thecooperation scheduler 190 notifies the result of the wireless resourceallocation as a cooperation scheduling result to the in-stationscheduler 105 (409), and ends the processing (410).

The flow chart of the processing of the cooperation scheduler 190 is anexample, and the cooperation scheduler 190 may process a cooperationscheduling request based on other scheduling standards.

The formats of packets necessary for data transmission in which the basestations 100 cooperate with one another are described next withreference to FIGS. 11A and 11B and FIGS. 12A and 12B.

FIG. 11A is a diagram illustrating the packet format of the cooperationinformation request signal according to the first embodiment of thisinvention.

A packet format 500 of the cooperation information request signalcontains a format identifier field 501, an allocated resource blockspecification field 502, an MCS field 503, a pilot locationspecification field 504, a power control field 505, a CQI request bitfield 506, a cooperation information request bit field 507, acooperation method field 508, a cooperation subband field 509, and anoption field 510.

The format identifier field 501 is a field for distinguishing the packetformat from other wireless resource allocation formats. The allocatedresource block specification field 502 is a field for specifying thelocation of an uplink resource block that is used to transmitcooperation information. A resource block in this case equals aplurality of consecutive SC-FDMA symbols and a plurality of consecutivesubcarriers in the SC-FDMA symbols, and uplink wireless resources areallocated on a resource block basis.

The MCS field 503 is a field for specifying a modulation and codingscheme, and MCS stands for Modulation and Coding Scheme. The pilotlocation specification field 504 is a field for information about wherea pilot is to be inserted by the destination wireless communicationterminal 200.

The power control field 505 is a field for information about powercontrol of the wireless communication terminal 200. The CQI request bitfield 506 is a field for requesting feedback of CQI from the wirelesscommunication terminal 200. Specifically, “1” is stored in the powercontrol field 505 in the case of a cooperation information requestsignal.

The cooperation information request bit field 507 is a field forindicating whether or not this signal is a cooperation informationrequest signal. Specifically, “1” is stored in the cooperationinformation request bit field 507 in the case of a cooperationinformation request signal.

The packet format 500 contains the cooperation method field 508 and thecooperation subband field 509 when “1” is stored in the cooperationinformation request bit field 507, and does not contain the cooperationmethod field 508 and the cooperation subband field 509 when “0” isstored in the cooperation information request bit field 507.

The cooperation method field 508 is used in the case where the senderbase station 100 or the cooperation scheduler 190 specifies atransmission method to be employed for cooperation. The cooperationsubband field 509 is used in the case where the sender base station 100or the cooperation scheduler 190 specifies a subband to be employed forcooperation. The option field 510 can be used for other expansions.

FIG. 11B is a diagram illustrating the packet format of the resourceallocation signal according to the first embodiment of this invention.

A packet format 520 of the resource allocation signal contains a formatidentifier field 521, an allocated resource block specification field522, a power control filed 523, a HARQ information field 524, atransport block-based information field 525, a cooperation informationfield 526, and a precoding information field 527.

The format identifier field 521 is a field for distinguishing the packetformat from other wireless resource allocation formats. The allocatedresource block specification field 522 is a field for specifying thelocation of a downlink resource block that is used to transmit data tothe destination wireless communication terminal 200 through cooperationamong the base stations 100. A resource block in this case equals aplurality of consecutive OFDMA symbols and a plurality of consecutivesubcarriers in the OFDMA symbols, and downlink wireless resources areallocated on a resource block basis.

The power control field 523 is a field for information about powercontrol of the wireless communication terminal 200. The HARQ informationfield 524 is a field for notifying a process number in Hybrid AutomaticRepeat Request (HARQ) transmission.

The packet format 520 contains as many transport block-based informationfields 525 as the number of transport blocks to be transmitted. Eachtransport block-based information field 525 contains an MCS field 525-1for specifying a modulation and coding scheme for each transport blockand a new HARQ field 525-2 for distinguishing whether or not thetransmission in question is new HARQ transmission.

The cooperation information field 526 is a field for information aboutwhich base stations participate in the cooperative transmission and whattransmission method is used in the cooperative transmission. Theprecoding information field 527 is a field for information about theindex and quantized values of a precoding matrix that is used in thecooperative transmission.

FIG. 12A is a diagram illustrating the packet format of a cooperationinformation notification signal for Open-Loop MIMO according to thefirst embodiment of this invention.

A packet format 530 of the cooperation information notification signalfor Open-Loop MIMO contains a cooperation method field 531, acooperating base station set field 532, a cooperation wideband CQI field533, and a subband-based information field 534.

The cooperation method field 531 is a field for specifying atransmission method to be used in the cooperative transmission. Thecooperating base station set field 532 is a field for notifying a set ofthe base stations 100 that participate in the cooperative transmission.The cooperation wideband CQI field 533 is a field for notifying CQI onevery OFDMA subcarrier in the cooperative transmission that uses themethod specified in the cooperation method field 531.

The packet format 530 contains as many subband-based information fields534 as the number of subbands. Each subband-based information field 534contains a cooperation subband CQI field 534-1 for storing the CQI ofeach subband in cooperative transmission, and a cooperation subband RIfield 534-2 for storing the RI of each subband in cooperativetransmission.

FIG. 12B is a diagram illustrating the packet format of a cooperationinformation notification signal for MU-MIMO according to the firstembodiment of this invention.

A packet format 540 of the cooperation information notification signalfor MU-MIMO contains a cooperation method field 541, a cooperating basestation set field 542, a cooperation wideband CQI field 543, and asubband-based information field 544.

The cooperation method field 541 is a field for specifying atransmission method to be used in the cooperative transmission. Thecooperating base station set field 542 is a field for notifying a set ofbase stations that participate in the cooperative transmission. Thecooperation wideband CQI field 543 is a field for notifying CQI on everyOFDMA subcarrier in the cooperative transmission that uses the methodspecified in the cooperation method field 541.

The packet format 540 contains as many subband-based information fields544 as the number of subbands. Each subband-based information field 544contains a channel matrix field 544-1 for storing values of a quantizedpropagation matrix between the base stations 100 that are specified inthe cooperating base station set field 542 and the wirelesscommunication terminals 200.

Described next with reference to FIGS. 1 and 13 is a sequence of datatransmission through MU-MIMO transmission in which the base stations 100cooperate with one another.

FIG. 13 is a sequence diagram illustrating processing form the receptionof pilot signals by the wireless communication terminals 200-1 to 200-6from the base stations 100-1 and 100-2 to the transmission ofinformation necessary for single-base station transmission by thewireless communication terminals 200-1 to 200-6 to the base stations100-1 and 100-2 according to the first embodiment of this invention.Each of the wireless communication terminals 200-1 to 200-6 belongs toone of the base stations 100-1 and 100-2, and transmits the necessaryinformation to the base station 100-1 or 100-2 to which the each of thewireless communication terminals 200-1 to 200-6 belongs.

By the time this sequence starts, the wireless communication terminals200-1 to 200-6 have obtained information necessary for data transmissionto the base stations 100-1 and 100-2, such as synchronizationinformation cell IDs of the base stations 100-1 and 100-2, through ananalysis of synchronization signals.

The wireless communication terminals 200-1 to 200-3 belong to the basestation 100-1, and the wireless communication terminals 200-4 to 200-6belong to the base station 100-2.

The base stations 100-1 and 100-2 periodically transmit pilot signals totheir respective wireless communication terminals 200-1 to 200-6 (621-1and 621-2). The wireless communication terminals 200-1 to 200-6 monitorthe received pilot signals and calculate channel matrices from the basestations 100-1 and 100-2.

When channel matrix calculation is performed in each of the wirelesscommunication terminals 200-1 to 200-6, interference components from thebase station to which the wireless communication terminal does notbelong (for example, the base station 100-2 in the case of the wirelesscommunication terminal 200-1) sometimes hinder the calculation. Theinterference components can be avoided by taking, for example, thefollowing measures.

Examples of known avoidance measures include one in which other basestations 100 are kept from transmitting data with a resource that isused by one base station 100 to transmit a pilot signal, and one inwhich SINR is improved by executing diffusion processing for a pilotsignal and then having the wireless communication terminal executeinverse diffusion. This invention is not limited to those measures andother avoidance measures may be employed.

Based on the calculated channel matrix, each of the wirelesscommunication terminals 200-1 to 200-6 calculates CQI, PMI, and RI insingle-base station transmission executed by the base station 100-1 or100-2 to which the each of the wireless communication terminals 200-1 to200-6 belongs, and feeds back the result of the calculation to the basestation 100-1 or 100-2 to which the each of the wireless communicationterminals 200-1 to 200-6 belongs via a control signal channel (622-1 to622-6).

In the example of FIG. 13, the wireless communication terminals 200-1 to200-3 feed back the calculation results to the base station 100-1, andthe wireless communication terminals 200-4 to 200-6 feed back thecalculation results to the base station 100-2.

FIG. 1 is a sequence diagram illustrating processing that is executedafter the sequence of FIG. 13, from the reception of user data destinedto the wireless communication terminals 200-1 to 200-6 by the basestations 100-1 and 100-2 from the gateway device 2, to the cooperativetransmission through MU-MIMO transmission according to the firstembodiment of this invention.

From the gateway device 2, the base station 100-1 receives user datadestined to the wireless communication terminals 200-1, 200-2, and 200-3(601-1), and the base station 100-2 receives user data destined to thewireless communication terminals 200-4, 200-5, and 200-6 (601-2).Receiving the user data, the in-station scheduler 105 of each of thebase stations 100-1 and 100-2 executes scheduling for single-basestation transmission.

At this point, the base stations 100-1 and 100-2 may exchange, via thecooperating base station IFs 107, information about Beam Forming (BF)beams such as a beam forming pattern (602) to be used for thescheduling.

The following description is given on the assumption that, as a resultof the scheduling for single-base station transmission, the base station100-1 has determined that wireless resource allocation for single-basestation transmission is possible with respect to the wirelesscommunication terminals 200-1 and 200-2, whereas the base station 100-2has determined that wireless resource allocation for single-base stationtransmission is possible with respect to the wireless communicationterminals 200-4 and 200-6.

In this case, the base station 100-1 transmits to the wirelesscommunication terminals 200-1 and 200-2, through single-base stationtransmission, resource allocation signals and user data destined to thewireless communication terminals 200-1 and 200-2 (603-1 and 603-2).

Receiving the signals given above, the wireless communication terminals200-1 and 200-2 transmit ACK signals indicating reception results to thebase station 100-1 in response (604-1 and 604-2).

Similarly, the base station 100-2 transmits to the wirelesscommunication terminals 200-4 and 200-6, through single-base stationtransmission, resource allocation signals for single-base stationtransmission and user data destined to the wireless communicationterminals 200-4 and 200-6 (603-4 and 603-6).

Receiving the signals given above, the wireless communication terminals200-4 and 200-6 transmit ACK signals indicating reception results to thebase station 100-2 in response (604-4 and 604-6).

Meanwhile, the base station 100-1 transmits a cooperation informationrequest signal to the wireless communication terminal 200-3 (605-3), andthe base station 100-2 transmits a cooperation information requestsignal to the wireless communication terminal 200-5 (605-5). Thecooperation information request signals to be transmitted have thepacket format of FIG. 11A.

Receiving the cooperation information request signal from the basestation 100-1, the wireless communication terminal 200-3 transmits acooperation information notification signal to the base station 100-1(606-3). Receiving the cooperation information request signal from thebase station 100-2, the wireless communication terminal 200-5 transmitsa cooperation information notification signal to the base station 100-2(606-5). The cooperation information notification signals to betransmitted have the packet format of FIG. 12B.

The base station 100-1 receives the cooperation information notificationsignal from the wireless communication terminal 200-3 and the basestation 100-2 receives the cooperation information notification signalfrom the wireless communication terminal 200-5. The base stations 100-1and 100-2 then separately issue requests for cooperation scheduling tothe cooperation scheduler 190 via the cooperation scheduler IFs 106(607-1 and 607-2). The requests contain the received cooperationinformation notification signals.

Based on the received cooperation information notification signals, thecooperation scheduler 190 allocates wireless resources for cooperativetransmission and, at the same time, refers to the result of the wirelessresource allocation to determine a transmission method to be used incooperative transmission.

In this embodiment, the cooperation scheduler 190 that has received therequest for cooperation scheduling allocates wireless resources forcooperative transmission. The following description assumes that, as aresult of the wireless resource allocation, the cooperation scheduler190 determines that cooperative transmission through MU-MIMOtransmission is to be performed for the wireless communication terminals200-3 and 200-5.

The cooperation scheduler 190 notifies the determination result andinformation necessary for cooperative transmission, such as a precodingmatrix, to the base stations 100-1 and 100-2 as a cooperation schedulingresult (608-1 and 608-2).

Receiving the cooperation scheduling result, the base stations 100-1 and100-2 exchange user data necessary for the cooperative transmissionspecified by the cooperation scheduling result via the cooperating basestation IFs 107 (609).

Based on the precoding matrices that are specified in the cooperationscheduling results 608-1 and 608-2, the base station 100-1 and the basestation 100-2 transmit resource allocation signals and user data to thewireless communication terminal 200-3 and the wireless communicationterminal 200-5, respectively, through MU-MIMO transmission such as DPC(610-3 and 610-5).

The wireless communication terminals 200-3 and 200-5 execute user datareception processing in a manner instructed in the resource allocationsignals upon receiving the resource allocation signal and the user data.Thereafter, the wireless communication terminal 200-3 transmits an ACKsignal that indicates the result of the reception to the base station100-1 in response (611-3), and the wireless communication terminal 200-5transmits an ACK signal that indicates the result of the reception tothe base station 100-2 in response (611-5).

According to the first embodiment of this invention, only the wirelesscommunication terminals 200-3 and 200-5 which need cooperativetransmission feed back cooperation information to the base stations100-1 and 100-2, thereby implementing cooperative transmission.

Uplink wireless resources are thus used more efficiently than in thecase where the wireless communication terminals 200-1 to 200-6 all feedback cooperation information, and can be saved for other uplink userdata transmission sessions.

In addition, processing required for cooperation scheduling is reducedbecause the cooperation scheduler 190 needs to execute cooperationscheduling processing only for the wireless communication terminals200-3 and 200-5.

Second Embodiment

A second embodiment of this invention is described below.

In the second embodiment of this invention, the network configurationand the configurations of the base stations 100 and the wirelesscommunication terminals 200 are the same as in the first embodiment ofthis invention, and descriptions thereof are therefore omitted. Thecomponents of each base station 100 and each wireless communicationterminal 200 in the second embodiment of this invention execute the sameprocessing as in the first embodiment, and therefore a descriptionthereof is also omitted.

The second embodiment is described below, with the focus on differencesfrom the first embodiment.

FIG. 14 is a sequence diagram illustrating processing form the receptionof user data destined to the wireless communication terminals 200-1 to200-6 by the base stations 100-1 and 100-2 from the gateway device 2, tocooperative transmission through SU transmission according to the secondembodiment of this invention.

In the second embodiment, which channels are used to transmit respectivesignals in communication between the base stations 100 and the wirelesscommunication terminals 200 that conforms to 3GPP LTE is also described.

The following description is given on the assumption that, prior to thissequence, the wireless communication terminals 200-1 to 200-6 havereceived pilot signals and information for single-base stationtransmission have been fed back to the base stations 100-1 and 100-2,following the sequence of FIG. 13.

The step in which the base stations 100-1 and 100-2 receive user data(601-1 and 601-2), the step in which the base stations 100-1 and 100-2exchange BF beam information (602), and the step in which the basestation 100-1 determines that wireless resource allocation forsingle-base station transmission is possible with respect to thewireless communication terminals 200-1 and 200-2, the base station 100-2determines that wireless resource allocation for single-base stationtransmission is possible with respect to the wireless communicationterminals 200-4 and 200-6, and the base stations 100-1 and 100-2transmit resource allocation signals and user data to these wirelesscommunication terminals 200 (603-1, 603-2, 603-4, and 603-6) are thesame as those in the first embodiment.

However, Downlink Control Information (DCI) on Physical Downlink ControlChannel (PDCCH) of LTE is used as the resource allocation signals. A DCIformat appropriate for the employed transmission method is selected.

The user data signals are transmitted on Physical Downlink SharedChannel (PDSCH).

The transmission of the ACK signals which are transmitted in response tothe resource allocation signals and the user data signals (604-1, 604-2,604-4, and 604-6) uses Physical Uplink Control Channel (PUCCH).

An expansion of DCI (format 0) on PDCCH to the packet format of FIG. 11Ais used for the cooperation information request signals (605-3 and605-5).

The wireless communication terminal 200-3 receives the cooperationinformation request signal (605-3) and transmits a cooperationinformation notification signal to the base station 101-1 (631-3). Thewireless communication terminal 200-5 receives the cooperationinformation request signal (605-5) and transmits a cooperationinformation notification signal to the base station 100-2 (631-5).

The cooperation information notification signals (631-3 and 631-5) havethe packet format of FIG. 12A, and are transmitted on Physical UplinkShared Channel (PUSCH).

Receiving the cooperation information notification signals (631-3 and631-5), the base station 100-1 and the base station 100-2 separatelyissue requests for cooperation scheduling to the cooperation scheduler190 via the cooperation scheduler IFs 106 (607-1 and 607-2). Therequests for cooperation scheduling contain the received cooperationinformation notification signals.

Receiving the cooperation scheduling requests, the cooperation scheduler190 allocates wireless resources for cooperative transmission.

The assumption of this embodiment is that, as a result of the wirelessresource allocation, the cooperation scheduler 190 has decided toexecute cooperative transmission through SU transmission for thewireless communication terminal 200-3, and to hold off wireless resourceallocation to the wireless communication terminal 200-5 until the nexttransmission timing.

The cooperation scheduler 190 notifies information necessary forcooperative transmission, including this decision, to the base stations100-1 and 100-2 as a cooperation scheduling result (632-1 and 632-2).

The base station 100-1 transfers to the base station 100-2, via thecooperating base station IF 107, user data necessary to carry out thespecified cooperative transmission upon receiving the cooperationscheduling result (633).

The base stations 100-1 and 100-2 transmit a resource allocation signal,and transmit user data through SU transmission such as Open-Loop MIMO,to the wireless connection terminal 200-3 (634-1 and 634-2).

It should be noted that, for the resource allocation signal, DCI onPDCCH is used. A DCI format appropriate for the employed transmissionmethod is selected. The user data signal is transmitted on PDSCH.

Receiving the resource allocation signal and the user data, the wirelesscommunication terminal 200-3 performs user data reception processing ina manner instructed in the resource allocation signal.

The wireless communication terminal 200-3 transmits an ACK signal thatindicates the result of the reception to the base station 100-1 inresponse (635).

According to the second embodiment of this invention, only the wirelesscommunication terminal 200-3 which needs cooperative transmission feedsback cooperation information, and uplink wireless resources are thussaved as in the first embodiment. This also reduces the processingexecuted by the cooperation scheduler 190 for cooperation scheduling.

In addition, in the second embodiment of this invention where thewireless communication terminal 200-3 alone uses two base stations 100-1and 100-2, the throughput in cooperative transmission is improved evenmore.

Third Embodiment

A third embodiment of this invention is described below.

In the third embodiment of this invention, the network configuration andthe configurations of the base stations 100 and the wirelesscommunication terminals 200 are the same as in the first embodiment ofthis invention, and descriptions thereof are therefore omitted. Thecomponents of each base station 100 and each wireless communicationterminal 200 in the third embodiment of this invention execute the sameprocessing as in the first embodiment, and therefore a descriptionthereof is also omitted.

The third embodiment is described below, with the focus on differencesfrom the first embodiment.

FIG. 15 is a sequence diagram illustrating processing that is executedin the case where the base stations 100-1 and 100-2 request informationnecessary for cooperative transmission before user data arrives at thewireless communication terminals 200 according to the third embodimentof this invention.

The following description is given on the assumption that, prior to thissequence, the wireless communication terminals 200-1 to 200-6 havereceived pilot signals and information for single-base stationtransmission have been fed back to the base stations 100-1 and 100-2,following the sequence of FIG. 13.

The assumption of this embodiment is that the base station 100-1 hasdetermined, from CQI information for single-base station transmission orfrom traffic at the wireless communication terminals 200-1 to 200-6,that cooperative transmission is to be used for the wirelesscommunication terminal 200-3, while the base station 100-2 has similarlydetermined that cooperative transmission is to be used for the wirelesscommunication terminal 200-5.

In this case, the base station 100-1 transmits a cooperation informationrequest signal to the wireless communication terminal 200-3 (641-3).Receiving the cooperation information request signal, the wirelesscommunication terminal 200-3 transmits a cooperation informationnotification signal to the base station 100-1 in response (642-3).

In the same manner, the base station 100-2 transmits a cooperationinformation request signal to the wireless communication terminal 200-5(641-5). Receiving the cooperation information request signal, thewireless communication terminal 200-5 transmits a cooperationinformation notification signal to the base station 100-2 in response(642-5).

In the case where the base stations 100-1 and 100-2 subsequently receiveuser data from the gateway device 2 (601-1 and 602-2), the base stations100-1 and 100-2 exchange BF beam information with each other (602), andtransmit cooperation scheduling requests (607-1 and 607-2) to thecooperation scheduler 190.

Receiving the cooperation scheduling requests, the cooperation scheduler190 notifies cooperation scheduling results to the base stations 100-1and 100-2 (608-1 and 608-2). Receiving the cooperation schedulingresults, the base stations 100-1 and 100-2 exchange informationnecessary for cooperation with each other (609), and transmit resourceallocation signals and user data to the wireless communication terminals200-1 to 200-6, respectively (643-1 to 643-6). Single-base stationtransmission is used for the wireless communication terminals 200-1,200-2, 200-4, and 200-6, and cooperative transmission is used for thewireless communication terminals 200-3 and 200-5.

Through the processing described above, data transmission to thewireless communication terminals 200-3 and 200-5, which need cooperativetransmission, can be synchronized with data transmission to the otherwireless communication terminals 200-1, 200-2, 200-4, and 200-6.

This allows the wireless communication terminals 200-1 to 200-6 totransmit ACK signals at the same time in response to received user data(644-1 to 644-6).

Wireless resource allocation in this embodiment is described below.

FIG. 18 is a diagram for explaining a resource grid upon cooperativetransmission according to the third embodiment of this invention.

The resource grid is a pattern obtained by sectioning wireless resourcesthat are allocated for one transmission timing by OFDMA symbol andsubcarrier.

In the example of FIG. 18, a horizontal grid line separates onesubcarrier from another and a vertical grid line separates one OFDMsymbol from another.

Resource girds 800 and 810 illustrated in FIG. 18 represent the resourcegrids of the base stations 100-1 and 100-2, respectively.

The resource grid 800 has a block 801, a block 802, and a block 803,which corresponds to a resource block 1, a resource block 2, and aresource block 3, respectively. The resource grid 810 has a block 811, ablock 812, and a block 813, which correspond to the resource block 1,the resource block 2, and the resource block 3, respectively.

In the sequence of FIG. 15, for example, the wireless communicationterminals 200-1 and 200-2 are allocated the resource blocks 801 and 802,respectively, and the wireless communication terminals 200-4 and 200-6are allocated the resource blocks 811 and 812, respectively.

The resource blocks 803 and 813 are allocated for cooperativetransmission that is executed for the wireless communication terminals200-3 and 200-5 through MU-MIMO transmission.

The resource blocks 801 to 803 and 811 to 813 are transmitted at thesame time in the sequence of FIG. 15.

According to the third embodiment of this invention, a delay intransmitting user data necessary for cooperative transmission iseliminated. Specifically, because the base stations 100-1 and 100-2 havealready obtained cooperation information by the time the base stations100-1 and 100-2 receive user data from the gateway device 2, the userdata can be transmitted immediately through cooperative transmissionwithout spending time on requesting cooperation information and waitingfor a cooperation information notification.

Fourth Embodiment

A fourth embodiment of this invention is described below.

In the fourth embodiment of this invention, the network configurationand the configurations of the base stations 100 and the wirelesscommunication terminals 200 are the same as in the first embodiment ofthis invention, and descriptions thereof are therefore omitted. Thecomponents of each base station 100 and each wireless communicationterminal 200 in the fourth embodiment of this invention execute the sameprocessing as in the first embodiment, and therefore a descriptionthereof is also omitted.

FIG. 16 is a sequence diagram illustrating processing that is executedwhen the wireless communication terminal 200 that has been at the cellcenter at first and has used single-base station transmission moves tothe cell edge and comes to need cooperative transmission according tothe fourth embodiment of this invention.

The base station 100-1 receives from the gateway device 2 user datadestined to the wireless communication terminal 200 (651), exchanges BFbeam information with the base station 100-2 if necessary (652), andallocates wireless resources for single-base station transmission.

In the case where the wireless resources for single-base stationtransmission are allocated as a result of the wireless resourceallocation, the base station 100-1 transmits a resource allocationsignal and user data to the wireless communication terminal 200 (653).

Receiving the resource allocation signal and the user data, the wirelesscommunication terminal 200 performs reception processing and transmitsan ACK signal to the base station 100-1 (654).

The wireless communication terminal 200 then moves to the cell edge(655).

The wireless communication terminal 200 calculates CQI and others basedon pilot signals (656-1 and 656-2) that are periodically transmittedfrom the base stations 100-1 and 100-2, and periodically feeds back thecalculated CQI and other values to the base station 100-1 as informationnecessary for single-base station transmission (657).

Receiving from the wireless communication terminal 200 the informationnecessary for single-base station transmission, the base station 100-1determines that sufficient wireless resources cannot be allocatedthrough data transmission from a single base station, and transmits acooperation information request signal to the wireless communicationterminal 200 (658).

Receiving the cooperation information request signal, the wirelesscommunication terminal 200 transmits a cooperation informationnotification signal to the base station 100-1 in response (659).

Receiving the cooperation information notification signal, the basestation 100-1 issues a request for cooperation scheduling to thecooperation scheduler 190 (660). The request for cooperation schedulingcontains the received cooperation information notification signal.

Receiving the request for cooperation scheduling, the cooperationscheduler 190 allocates wireless resources to the wireless communicationterminal 200 for cooperative transmission through SU transmission, andnotifies the result of the wireless resource allocation to the basestations 100-1 and 100-2 as a cooperation scheduling result (661-1 and661-2).

The base station 100-1 receives the cooperation scheduling result andtransfers user data necessary for the cooperative transmission to thebase station 100-2 (662).

The base station 100-1 and 100-2 transmit resource allocation signals,and transmit user data by cooperative transmission through SUtransmission, to the wireless communication terminal 200 (663-1 and663-2).

Receiving the resource allocation signals and the user data, thewireless communication terminal 200 performs reception processing andtransmits an ACK signal to the base station 100-1 in response (664).

According to the fourth embodiment of this invention, the wirelesscommunication terminal 200 needs to transmit cooperation informationonly when the wireless communication terminal 200 moves to a place thatnecessitates cooperative transmission, and uplink wireless resources cantherefore be saved.

Fifth Embodiment

A fifth embodiment of this invention is described below.

In the fifth embodiment of this invention, the network configuration andthe configurations of the base stations 100 and the wirelesscommunication terminals 200 are the same as in the first embodiment ofthis invention, and descriptions thereof are therefore omitted. Thecomponents of each base station 100 and each wireless communicationterminal 200 in the fifth embodiment of this invention execute the sameprocessing as in the first embodiment, and therefore a descriptionthereof is also omitted.

FIG. 17 is a sequence diagram illustrating processing that is executedwhen the wireless communication terminal 200 that has at the cell edgeat first and has used data transmission by cooperating transmissionmoves to the cell center and switches to single-base stationtransmission according to the fifth embodiment of this invention.

The base station 100-1 receives from the gateway device 2 user datadestined to the wireless connection terminal 200 (671), and transfersthe user data to the base station 100-2 in order to execute cooperativetransmission (672).

The base station 100-1 and 100-2 transmit resource allocation signals,and transmit user data by cooperative transmission through SUtransmission, to the wireless communication terminal 200 (673-1 and673-2).

Receiving the resource allocation signals and the user data, thewireless communication terminal 200 performs reception processing andtransmits an ACK signal to the base station 100-1 in response (674).

The wireless communication terminal 200 then moves to the cell center(675).

The wireless communication terminal 200 calculates CQI and others basedon pilot signals (676-1 and 676-2) that are periodically transmittedfrom the base stations 100-1 and 100-2, and periodically feeds back thecalculated CQI and other values as information necessary for single-basestation transmission (677).

The base station 100-1 receives the information from the wirelesscommunication terminal 200 and, if necessary, exchanges BF beaminformation with the base station 100-2 (678).

The assumption of this embodiment is that the base station 100-1determines that sufficient wireless resources can be allocated throughdata transmission from a single base station.

The base station 100-1 in this case notifies, if necessary, thecooperation scheduler 190 of the cessation of cooperative transmissionto the wireless communication terminal 200 (679).

The base station 100-1 transmits resource allocation signals, andtransmits user data by single-base station transmission, to the wirelesscommunication terminal 200 (680).

Receiving the resource allocation signals and the user data, thewireless communication terminal 200 performs reception processing andtransmits an ACK signal to the base station 100-1 in response (681).

According to the fifth embodiment of this invention, the wirelesscommunication terminal ceases to transmit cooperation information whenthe wireless communication terminal 200 moves to a place that does notnecessitate cooperative transmission, and uplink wireless resources cantherefore be saved.

Sixth Embodiment

A sixth embodiment of this invention is described below.

The sixth embodiment describes a method of reducing the amount ofcooperation information that is transmitted from the wirelesscommunication terminals 200 in the first to fifth embodiments describedabove by using a preselected subband fixedly to feed back cooperationinformation.

FIG. 19A is a diagram illustrating subbands according to the sixthembodiment of this invention.

As illustrated in FIG. 19A, subbands are constituted of consecutiveresource blocks.

A resource grid 820 in the example of FIG. 19A is divided into fivesubbands 821 to 825, each of which is constituted of two consecutiveresource blocks.

FIG. 19B is a diagram illustrating an association relation between anumber of the resource block and a number of the subband according tothe sixth embodiment of this invention.

The number of resource blocks per subband can be changed by changing thetotal number of resource blocks.

As illustrated in FIG. 19B, a table representing the associationrelation between the number of the resource block and the number of thesubband includes in each entry a number of a total resource block 561, asubband size 562, and a number of a subband 563.

The number of the total resource block 561 includes the total number ofresource blocks. The subband size 562 includes the number of resourceblocks per subband. The number of the subband 563 includes the totalnumber of subbands. The example of FIG. 19A corresponds to an entry 564of FIG. 19B.

In the examples of FIGS. 12A and 12B, a cooperation informationnotification signal fed back by the wireless communication terminals 200needs to contain as many pieces of cooperation information as the numberof subbands. Using a preselected subband fixedly for cooperativetransmission in the wireless communication system reduces the amount ofcooperation information contained in the cooperation informationnotification signal and accordingly saves uplink wireless resources.

For example, in the case where two subbands, the subband 824 and thesubband 825, are used fixedly for cooperative transmission in FIG. 19A,necessary cooperation information is ⅖ of the amount required when adifferent subband is used each time.

Instead of using a preselected subband fixedly for cooperativetransmission, the cooperation scheduler 190 may dynamically determine asubband optimum for cooperative transmission. The determined subband iscontained in the cooperation subband field 509 within the cooperationinformation request signal as illustrated in FIG. 11A, thereby allowingthe base station 100 to give the destination wireless communicationterminal 200 an instruction about which subband to use.

According the sixth embodiment of this invention, uplink wirelessresources for feeding back cooperation information can be saved and, ifthe option of dynamically selecting a subband is chosen, a subbandoptimum for cooperative transmission can be selected.

Seventh Embodiment

A seventh embodiment is described below.

The seventh embodiment describes a method of using NACK for a switchbetween data transmission through single-base station transmission anddata transmission through cooperative transmission.

In the embodiments described above, whether or not to performcooperative transmission is determined based on the success/failure ofdownlink wireless resource allocation or on CQI, PMI, and RI. However,other standards such as the following ones may be used for thedetermination:

Each base station 100 decides to use cooperative transmission for thewireless communication terminal 200 that has used single-base stationtransmission if an NACK signal indicating a reception failure isreceived from the wireless communication terminal 200 a reference numberof times, which is determined in advance, or more. In other words,whether or not to perform cooperative transmission is determined basedon information about the success/failure of data reception which istransmitted from the wireless communication terminal 200.

According to the seventh embodiment, the base stations 100 can salvagedata that has failed to be received by determining whether or not to usecooperative transmission based on the actual success/failure of datareception.

Eighth Embodiment

An eighth embodiment is described below.

The eighth embodiment describes a method in which each base station 100transmits all cooperation information request signals at once. In thefirst to seventh embodiments, the base station 100 separately transmitsa cooperation information request signal destined to one wirelesscommunication terminal 200 and a cooperation information request signaldestined to another wireless communication terminal 200. In thisembodiment, on the other hand, the base station 100 transmitscooperation information request signals through one-to-many transmissionsuch as broadcast transmission or multicast transmission. For example,in the case where the number of wireless communication terminals 200that cannot be allocated wireless resources through single-base stationtransmission exceeds a certain threshold, the base station 100 transmitscooperation information request signals to these wireless communicationterminals 200 at once. The wireless communication terminals 200 receivethe cooperation information request signals transmitted at once andtransmit cooperation information notification signals to the basestation 100.

According to the eighth embodiment, there is obtained an effect of beingable to save downlink wireless resources necessary for cooperationinformation request signals when the number of wireless communicationterminals 200 that need cooperative transmission is large.

Lastly, effects of this invention are described taking as an example anassociation relation between the ratio of the wireless communicationterminals 200 that use cooperative transmission to all wirelesscommunication terminals 200 (hereinafter, referred to as cooperatingwireless communication terminal ratio) and the amount of feedbacktransmitted by a single wireless communication terminal 200 when thefirst embodiment or second embodiment of this invention is applied. Thesame effects are obtained from the third to eighth embodiment of thisinvention.

FIG. 20 is a diagram illustrating effects that are obtained when thefirst embodiment or second embodiment of this invention is applied.

A frame 6000 illustrates information that is fed back from the wirelesscommunication terminals 200 to the base stations 100 when conventionalcooperative transmission is executed. Conventionally, as illustrated inthe frame 6000, all wireless communication terminals 200-1 to 200-6periodically feed back information for single-base station transmissionand cooperation information to the base stations 100-1 and 100-2 (625-1to 625-6).

A frame 6001 illustrates information that is fed back from the wirelesscommunication terminals 200 to the base stations 100 when the firstembodiment or second embodiment of this invention is applied.

According to this invention, only the wireless communication terminals200-3 and 200-5 which need cooperative transmission feed backcooperation information to the base stations 100-1 and 100-2 in responseto requests from the base stations 100-1 and 100-2 (606-3 and 606-5).The only information the wireless communication terminals 200-1 to 200-6need to feed back periodically to the base stations 100-1 and 100-2 isinformation for single-base station transmission (622-1 to 622-6). Theamount of feedback is reduced as a result.

FIG. 21A is a graph illustrating changes in the number of bits in anuplink wireless resource band that is used by one wireless communicationterminal 200 for one feedback transmission session in the case where thecooperating wireless communication terminal ratio is changed based onthe packet format of the cooperation information notification signal ofFIG. 12A.

It should be noted that, in the following description, the cooperationmethod field 531 and the cooperating base station set field 532 togethertake up four bits in the packet format of the cooperation informationnotification signal. The cooperation wideband CQI field 533 uses fourbits. The cooperation subband CQI field 534-1 and the cooperationsubband RI field 534-2 combined use three bits.

Graphs 901, 902, 903, and 904 of FIG. 21A respectively represent resultsobserved when the number of the subband is 5, 13, 21, and 28.

Values at a cooperating wireless communication terminal ratio of “1.0”represent results of the conventional case. It is understood from FIG.21A that applying this invention decreases the number of the feedbackbit.

FIG. 21B is a graph illustrating changes in the number of bits in anuplink wireless resource band that is used by one wireless communicationterminal 200 for one feedback transmission session in the case where thecooperating wireless communication terminal ratio is changed based onthe packet format of the cooperation information notification signal ofFIG. 12B.

It should be noted that, in the following description, the cooperationmethod field 541 and the cooperating base station set field 542 togethertake up four bits in the packet format of the cooperation informationnotification signal. The cooperation wideband CQI field 543 uses fourbits. The channel matrix field 544-1 uses twelve (6×2) bits. Graphs 911,912, 913, and 914 of FIG. 21B respectively represent results observedwhen the number of the subband is 5, 13, 21, and 28.

Values at a cooperating wireless communication terminal ratio of “1.0”represent results of the conventional case. It is understood from FIG.21B that applying this invention decreases the number of the feedbackbit.

1. A wireless communication system comprising a plurality of basestations that transmit data to a communication terminal with cooperationby the plurality of base stations, wherein the communication terminalcommunicates with the plurality of base stations, the communicationterminal periodically transmits, to one of the plurality of basestations, information necessary for data transmission from a single basestation out of the plurality of base stations, each of the plurality ofbase stations is configured to: determine whether the communicationterminal needs data transmission through a cooperation among theplurality of base stations; and transmit a cooperation informationtransmission instruction to the communication terminal, which includesinformation necessary to execute the data transmission in order tocooperate among the plurality of base stations in the case where it isdetermined that the communication terminal needs the data transmissionthrough the cooperation among the plurality of base stations, and thecommunication terminal transmits the cooperation information to theplurality of base stations in a case of receiving the cooperationinformation transmission instruction.
 2. The wireless communicationsystem according to claim 1, wherein the each of the plurality of basestations is configured to: determine whether the data transmission fromthe single base station is executable for the communication terminal inthe case where determining whether the communication terminal needs thedata transmission through the cooperation among the plurality of basestations; transmit the cooperation information transmission instructionto the communication terminal in the case where it is determined thatthe data transmission from the single base station is not executable forthe communication terminal; and allocate wireless resources for the datatransmission from the single base station to the communication terminal,based on the received information necessary for the data transmissionfrom the single base station in the case where it is determined that thedata transmission from the single base station is executable for thecommunication terminal.
 3. The wireless communication system accordingto claim 2, further comprising a cooperation scheduler, which isconfigured to: allocate the communication terminal wireless resourcesfor the data transmission through the cooperation among the plurality ofbase stations, based on the cooperation information received by theplurality of base stations; and transmit a result of the wirelessresource allocation for the data transmission through the cooperationamong the plurality of base stations to the plurality of cooperatingbase stations.
 4. The wireless communication system according to claim3, wherein, the communication terminal transmits the cooperationinformation to the plurality of base stations which includes informationnecessary to execute data transmission to one communication terminalthrough the cooperation among the plurality of base stations, andinformation necessary to execute data transmission to a plurality of thecommunication terminals through the cooperation among the plurality ofbase stations, and the each of the plurality of base stations isconfigured to determine whether the data transmission to the onecommunication terminal through the cooperation among the plurality ofbase stations or the data transmission to the plurality of communicationterminals through the cooperation among the plurality of base stationsis to be executed based on the received cooperation information.
 5. Thewireless communication system according to claim 3, wherein, the each ofthe plurality of base stations is configured to: determine which ofinformation necessary to execute data transmission to one communicationterminal through the cooperation among the plurality of base stationsand information necessary to execute data transmission to a plurality ofthe communication terminals through the cooperation among the pluralityof base stations is needed, and transmit the cooperation informationtransmission instruction based on the determination.
 6. The wirelesscommunication system according to claim 3, wherein, the wirelessresources are divided into a plurality of subchannels, the cooperationscheduler is configured to allocate a predetermined specific subchannelto the communication terminal as the wireless resources for the datatransmission through the cooperation among the plurality of basestations, and the base station is configured to set information aboutthe predetermined specific subchannel in the cooperation informationtransmission instruction.
 7. The wireless communication system accordingto claim 3, wherein, the wireless resources are divided into a pluralityof subchannels, the cooperation scheduler is configured to determine atleast one of the plurality of subchannels which is to be allocated asthe wireless resources for the data transmission through the cooperationamong the plurality of base stations, and the base station is configuredto transmit the cooperation information transmission instruction whichincludes information about the determined subchannel.
 8. The wirelesscommunication system according to claim 2, wherein the each of theplurality of base stations is configured to refer to a result of thewireless resource allocation to the communication terminal for the datatransmission from the single base station, and determine whether thedata transmission from the single base station is executable for thecommunication terminal.
 9. The wireless communication system accordingto claim 2, wherein the each of the plurality of base stations isconfigured to refer to the information necessary for the datatransmission from the single base station which is transmitted from thecommunication terminal, and determine whether the data transmission fromthe single base station is executable for the communication terminal.10. The wireless communication system according to claim 2, wherein theeach of the plurality of base stations is configured to refer to trafficinformation of the communication terminal, and determine whether thedata transmission from the single base station is executable for thecommunication terminal.
 11. The wireless communication system accordingto claim 2, wherein the each of the plurality of base stations isconfigured to refer to ACK information and NACK information which aretransmitted from the communication terminal, and determine whether thedata transmission from the single base station is executable for thecommunication terminal.
 12. The wireless communication system accordingto claim 2, wherein the cooperation information includes at least one ofa channel quality, the number of MIMO ranks, a precoding matrix desiredby the communication terminal, and a channel matrix in the datatransmission through the cooperation among the plurality of basestations.
 13. A base station installed in a wireless communicationsystem that transmits data to a communication terminal throughcooperation among a plurality of the base stations, wherein, thecommunication terminal communicates with the base station, thecommunication terminal periodically transmits, to the base station,information necessary for data transmission from a single base stationout of the plurality of base stations, the base station is configuredto: determine whether the communication terminal needs data transmissionthrough the cooperation among the plurality of base stations; andtransmit a cooperation information transmission instruction to thecommunication terminal, which includes information necessary to executethe data transmission order to cooperate among the plurality of basestations in the case where it is determined that the communicationterminal needs the data transmission through the cooperation among theplurality of base stations.
 14. The base station according to claim 13,wherein the base station is further configured to: determine whether thedata transmission from the single base station is executable for thecommunication terminal in the case where determining whether thecommunication terminal needs the data transmission through thecooperation among the plurality of base stations; transmit thecooperation information transmission instruction to the communicationterminal in the case where it is determined that the data transmissionfrom the single base station is not executable for the communicationterminal; receive the cooperation information transmitted from thecommunication terminal; allocate wireless resources for the datatransmission through the cooperation among the plurality of basestations based on the received cooperation information; transmit aresult of the wireless resource allocation for the data transmissionthrough the cooperation among the plurality of base stations to theother base stations that cooperate with the base station; and allocatewireless resources for the data transmission from the single basestation to the communication terminal, based on the received informationnecessary for the data transmission from the single base station in thecase where it is determined that the data transmission from the singlebase station is executable for the communication terminal.
 15. The basestation according to claim 14, wherein the cooperation informationincludes information necessary to execute data transmission to onecommunication terminal through the cooperation among the plurality ofbase stations, and information necessary to execute data transmission toa plurality of the communication terminals through the cooperation amongthe plurality of base stations, and wherein, the base station isconfigured to determine whether the data transmission to the onecommunication terminal through the cooperation among the plurality ofbase stations or the data transmission to the plurality of communicationterminals through the cooperation among the plurality of base stationsis to be executed based on the received cooperation information. 16.(canceled)